1. The ornamental design for a 3D printing pen, as shown and described.

Description:

FIG. 1 is a perspective view of a 3D printing pen showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top view thereof;

FIG. 7 is a bottom view thereof; and,

FIG. 8 is the second perspective view showing the 3D printing pen.

The evenly spaced broken lines in the drawings represent unclaimed subject matter and form no part of the claimed design. The long and short broken lines in the drawings that define the bounds of the claimed design form no part thereof.